Doors in aircraft must on the one hand be able to be closed tightly and must be able to accommodate securely all loads that occur, and on the other hand must be quick and easy to open in an emergency. There are essentially two principles in accordance with which, for example, locking mechanisms for aeroplane doors can be designed.
A door designed in accordance with the so-called abutment principle has abutment fittings on its lateral edges, which in the closed position of the door abut against corresponding fuselage-side fittings such that a form fit is formed between the door and the fuselage, via which the internal forces acting on the door are transferred to the fuselage structure. A door of this type is e.g. shown in the patent document U.S. Pat. No. 4,720,065 and must be lowered for purposes of closing and raised for purposes of opening, in order to release the abutment fittings for an opening movement that is directed outwards.
A door designed in accordance with the so-called toggle principle has on its lateral edges toggles that can rotate; as the door pivots inwards these stand parallel to the pivoting movement and engage in fuselage-side holding fixtures. For purposes of closing the door the toggles are set transverse within the holding fixtures, so that here too a form fit is achieved.
However, what is disadvantageous for both principles is the high level of complexity of the kinematic system required, since a large number of shafts, bearings, levers, linkages, crank drives and similar are necessary.
Moreover, there are aeroplane doors that are designed in accordance with both the abutment principle and the toggle principle. The combination of the two principles leads, however, less to a reduction of complexity than to an increase of the same, and thus, in addition to an increase in weight, to relatively high manufacturing costs.